1. Field of the Invention
This invention relates to on-line line monitor systems used for monitoring ATM cross-connect equipments, which perform cross-connecting with respect to ATM cells (where "ATM" is an abbreviation for "Asynchronous Transfer Mode"). Particularly, this invention relates to the on-line line monitor system that performs fault diagnosis with respect to circuit blocks provided for the cross-connecting. This application is based on patent application No. Hei 9-178103 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
The communication networks having redundancy configurations provide presently operating lines (or presently activated lines which are presently placed in an on-line state; hereinafter, simply referred to as operating lines) and spare lines (which are not presently used and are placed in an off-line state or a standby state). When faults occur on the operating lines, the communication networks switch over the operating lines to the spare lines. In an event of occurrence of a large fault in the above communication network, the operating line is subjected to shutoff, so that an input shutoff event occurs. In such an event, the communication network uses the fault as a trigger to switch over line control from the operating line to the spare line.
FIG. 8 shows a conventional example of the on-line line monitor system, which is disclosed by the paper of Japanese Patent Application, Publication No. Hei 4-51723. Herein, there are provided a first line switching equipment 13 and a second line switching equipment 14, which are connected together by an operating line 11 and a spare line 12. The second line switching equipment 14 contains a test signal generation circuit 16 and a test signal detection circuit 17. Herein, the test signal generation circuit 16 sends test signals to a first terminal of the spare line 12, while the test signal detection circuit 17 receives the test signals returned to a second terminal of the spare line 12 so as to detect a state of the spare line 12. In addition, the second line switching equipment 14 contains switched connections 18 to 21. The switched connection 18 is controlled by a first switch (not shown) that performs switching of the line for transmission and reception of the test signals, while the switched connections 19 to 21 are controlled by a third switch (not shown) that performs switchover between the operating line 11 and the spare line 12.
The first line switching equipment 13 provides a second switch (not shown) that switches over the line used for transmission and reception of the test signals in response to the first switch, as well as a fourth switch (not shown) that switches over line control between the operating line 11 and the spare line 12. In addition, a loopback circuit 24 having a switched connection 23 is provided for the second switch to turn back the test signals, while the fourth switch has switched connections 25 to 27.
In the aforementioned on-line line monitor system normally uses the operating line 11. For this reason, the aforementioned switches establish connections shown in FIG. 8. That is, the fourth switch of the first line switching equipment 13 establishes connections between the switched connections 25 and 26, while the third switch of the second line switching equipment 14 establishes connections between the switched connections 19 and 20. With respect to the spare line 12, the second switch of the first line switching equipment 13 establishes connections between the switched connections 23 and 27, while the first switch of the second line switching equipment 14 establishes connections between the switched connections 18 and 21. Thus, the spare line 12 is placed in a loopback state in the first line switching equipment 13, so that test signals, which are generated by the test signal generation circuit 16 of the second line switching equipment 14 and are transmitted onto the spare line 12, are returned back to the test signal detection circuit 17. The test signal detection circuit 17 examines quality of transmission characteristics of the spare line 12. In the case of defective, the on-line line monitor system gives warning. In this case, a maintenance man repairs the spare line 12, so that the spare line 12 can be normally retained in a normal state.
FIG. 9 shows connections that take place when a fault occurs on the operating line 11 so that the on-line line monitor system switches over the operating line 11 to the spare line 12, wherein parts identical to those of FIG. 8 are designated by the same reference symbols. As shown in FIG. 9, the on-line line monitor system establishes connections between the operating line 11, the test signal generation circuit 16 and the test signal detection circuit 17. Thus, the maintenance man is capable of knowing characteristics of the operating line 11 which causes the fault.
FIG. 10 shows another conventional example of the on-line line monitor system, which is disclosed by the paper of Japanese Patent Application, Publication No. Sho 62-279752. This system is applied to the double loop optical communication network, which is configured by double loop circuits (hereinafter, referred to as 0-loop and 1-loop respectively). There are provided a 0-loop light transmission path 40 and a 1-loop light transmission path 41, which correspond to light loops. A central control unit 31 contains a system control section 32 which is provided for both of the light loops, as well as a central control section 33 for the 0-loop and a central control section 34 for the 1-loop. In addition, the double loop optical communication network of FIG. 10 contains terminal devices 35, 36 and 37, which are arranged along the light loops in connection with the central control unit 31. So, the central control unit 31 is connected to the terminal devices 35 to 37 by means of the 0-light-transmission path 40 and the 1-light-transmission path 41. The terminal devices 35, 36 and 37 contain terminal control sections 43, 44 and 45 for the 0-loop as well as terminal control sections 46, 47 and 48 for the 1-loop respectively. The central control section 33 and the terminal control sections 43 to 45, all of which are provided for the 0-loop, are connected together in loop configuration by means of the 0-loop light transmission path 40. In addition, the central control section 34 and the terminal control sections 46 to 48, all of which are provided for the 1-loop, are connected together in loop configuration by means of the 1-loop light transmission path 41. Transmission direction of the 0-loop light transmission path 40 is reverse to that of the 1-loop light transmission path 41. The central control unit 31 subjects a prescribed bit pattern of a specific channel to inverse double transmission, so that the central control unit 31 normally sends the bit pattern on the light transmission paths.
FIG. 11 shows an error event that a bit error occurs on a certain light transmission path, wherein parts identical to those of FIG. 10 are designated by the same reference symbols. In FIG. 11, a mark of "X" indicates a fault location 51, which is placed between the terminal devices 35 and 36. Either the terminal device 35 or the terminal device 36 detects occurrence of malfunction, which continuously occurs R times or more, with respect to the bit pattern output from the central control device 31. That is, the terminal control section 44 of the terminal device 36 detects malfunction that occurs on the 0-loop light transmission path 40, while the terminal control section 46 of the terminal device 35 detects malfunction that occurs on the 1-loop light transmission path 41. Upon detection of the malfunction, either the terminal device 35 or the terminal device 36 performs loopback operation. Further, the system control section 32 of the central control unit 31 issues a bypass instruction or a loopback instruction to the central control section 33 for the 0-loop and the central control section 34 for the 1-loop, so that as shown by dotted lines, the double loop configuration is changed to a single loop configuration.
The aforementioned on-line line monitor systems shown in FIG. 8, FIG. 9, FIG. 10 and FIG. 11 are designed as follows:
When some large fault occurs on the operating line, the system uses such a fault as a trigger to switch over line control from the operating line to the spare line.
However, the aforementioned systems do not employ a method that test signals are incorporated into signals transmitted on the operating line, which is presently used for the communication service, so as to make determination as to whether the operating line has a fault or not. Because, the conventional communication systems mainly uses STM (an abbreviation for "Synchronous Transfer Mode"). So, it is impossible to incorporate the test signals into actual communication signals without breaking the communication services.
As the method to check the operating line, the conventional system employs a method that adds a parity to signals, which are processed by every unit of byte. According to this method, it is possible to detect occurrence of error in the signals by the parity check that is performed at a reception side to receive the signals. In this method, in which a parity bit is configured by a single bit, however, if error corresponding to two bits in total occurs on the line, it is impossible to make determination as to whether the line has a fault or not. In addition, it is impossible to perform fault diagnosis with respect to all the circuits by merely using the parity check. For this reason, it can be said that the parity check is insufficient to make determination as to whether a fault occurs in the circuit operation or not. Further, if a number of parity bits is increased, the system suffers from a problem that a circuit portion for processing communication information should be enlarged in scale in response to an increase of the parity bits.